Centrifugal compressor

ABSTRACT

A centrifugal compressor for compressing a fluid in a gas phase or a supercritical phase includes: a rotational shaft; an axial flow path extending along an axial direction of the centrifugal compressor; a radial flow path communicating with the axial flow path and extending along a radial direction of the centrifugal compressor on a downstream side of the axial flow path; an impeller at least partially disposed in the radial flow path and configured to rotate together with the rotational shaft to increase pressure of the fluid flowing in the radial flow path; and a pre-compression unit disposed in the axial flow path at a position distant from a leading edge of the impeller on an upstream side of the leading edge and configured to increase the pressure of the fluid in advance before the fluid is introduced to the leading edge.

TECHNICAL FIELD

The present disclosure relates to a centrifugal compressor for compressing a fluid in a gas phase or a supercritical phase.

BACKGROUND

In conventionally known centrifugal compressors, a fluid is mainly compressed by centrifugal force while flowing in a radial direction due to the rotation of an impeller. For example, the centrifugal compressors have been used in various plants including a chemical plant, a gas turbine plant, and a refrigerator.

For example, Patent Document 1 discloses a centrifugal compressor including a plurality of impellers arranged around a main shaft. Patent Document 2 discloses a gas compressor including a centrifugal rotor including an impeller (centrifugal flow blade).

CITATION LIST Patent Literature Patent Document 1: Japanese Patent Application Laid-open No. 2002-21784 Patent Document 2: Japanese Translation of PCT Application No. 2004-516401 SUMMARY Technical Problem

The centrifugal compressors are required to maintain a high compression efficiency. Power required for driving the compressor can be largely reduced when an inlet temperature of a fluid can be reduced without compromising the high efficiency of the centrifugal compressor. However, when a fluid temperature is low, portions at or below saturation pressure might be locally generated in the compressor, and the resultant partial condensation might largely compromise the performance of the compressor. More specifically, the performance of the compressor might be largely compromised by droplets, generated by the condensation, spreading due to centrifugal force to block a flow path.

In view of the above, an object of at least one embodiment of the present invention is to provide a centrifugal compressor facilitating an attempt to improve a compression efficiency while preventing the partial condensation in the compressor.

Solution to Problem

As a result of vigorous studies, the present inventors have found out that the partial condensation, as a factor of degrading the performance of the centrifugal compressor, is likely to occur in a flow path on an inlet side of an impeller. Furthermore, as a result of CFD, the present inventors have found out that a flow rate tends to increase in the vicinity of a leading edge of the impeller, that is, in an area of the leading edge in the vicinity of a suction surface in particular. Based on this tendency, it has been found that static pressure, around the impeller, is the lowest in the vicinity of the leading edge of the impeller (more specifically, in the vicinity of the leading edge on the suction surface of the impeller), while the centrifugal compressor is operating. Thus, a risk of partial condensation is high in this portion.

All things considered, it is important to prevent the partial condensation from occurring in a leading edge portion of the impeller to improve the compression efficiency of the centrifugal compressor.

A centrifugal compressor according to the at least one embodiment of the present invention, based on the above findings made by the present inventors, is a centrifugal compressor for compressing a fluid in a gas phase or a supercritical phase, the centrifugal compressor including:

a rotational shaft;

an axial flow path extending along an axial direction of the centrifugal compressor;

a radial flow path communicating with the axial flow path and extending along a radial direction of the centrifugal compressor on a downstream side of the axial flow path;

an impeller at least partially disposed in the radial flow path and configured to rotate together with the rotational shaft to increase pressure of the fluid flowing in the radial flow path; and

a pre-compression unit disposed in the axial flow path at a position distant from a leading edge of the impeller on an upstream side of the leading edge and configured to increase the pressure of the fluid in advance before the fluid is introduced to the leading edge.

In the centrifugal compressor, the pre-compression unit is disposed in the axial flow path at the position on the upstream side of the leading edge, and pre-compresses the fluid. Thus, the pressure of the fluid can be easily maintained at or higher than the saturation pressure in the vicinity of the leading edge of the impeller, whereby the partial condensation can be prevented from occurring. Thus, a high compression efficiency can be maintained while preventing the degradation of the compression performance.

The partial condensation might occur in the pre-compression unit disposed in the axial flow path. Still, the centrifugal compressor is less affected by such partial condensation compared with the partial condensation occurring in the radial flow path for the following reason. Specifically, the flow direction of the fluid and the direction of the centrifugal force are different from each other in the axial flow path. Thus, droplets as a result of the partial condensation in the axial flow path would not spread over the entire axial flow path due to the centrifugal force. On the other hand, droplets as a result of the partial condensation on the inlet side of the radial flow path would spread over the entire radial flow path due to the centrifugal force, and might block the flow path.

Thus, even when the inlet temperature of the centrifugal compressor is low, the pressure in the vicinity of the leading edge of the impeller can be maintained at or higher than the saturation pressure with the pre-compression unit disposed in the axial flow path increasing the pressure of (pre-compressing) the fluid, whereby the partial condensation can be prevented from occurring. Thus, the centrifugal compressor can be operated under an operation condition with a lower inlet temperature (an operation condition which has had a high risk of partial condensation in conventional configurations). Thus, the compression efficiency can be improved.

In some embodiments, the centrifugal compressor is a multi-stage compressor including at least one section including a plurality of the impellers arranged on multiple stages along a flow direction of the fluid, and

the pre-compression unit is disposed in the axial flow path on an upstream side of a first-stage impeller in each of the at least one section, at a position distant from a leading edge of the first-stage impeller on an upstream side of the first-stage impeller.

In the multi-stage compressor, the flow path in the vicinity of the first-stage impeller has a lower pressure than flow paths in the vicinity of other impellers, and thus is regarded as having a highest risk of the partial condensation. Thus, as in the embodiment described above, the pre-compression unit is disposed in the axial flow path on the upstream side of the first-stage impeller, and pre-compresses the fluid before flowing into the radial flow path in the vicinity of the leading edge of the first-stage impeller. Thus, the partial condensation can be effectively prevented in the area (in the vicinity of the leading edge of the first-stage impeller) where the condensation is most likely to occur.

In some embodiments, the pre-compression unit is configured to rotate together with the rotational shaft to increase the pressure of the fluid. In one embodiment, the pre-compression unit includes a spiral blade disposed on an outer circumference side of the rotational shaft and extending along the axial direction spirally around the rotational shaft.

In the embodiment described above, when the rotational shaft rotates, the pre-compression unit (spiral blade) rotates so that the fluid in the axial flow path is guided toward the radial flow path while having the pressure increased. With the spiral blade thus used for the pre-compression unit, the pressure of the fluid can be increased by the driving force of the rotational shaft, whereby a simple device configuration can be achieved.

In one embodiment, the pre-compression unit further includes a shroud disposed on an outer circumference side of the spiral blade so as to cover the spiral blade.

In the embodiment described above, the shroud disposed on the outer circumference side of the spiral blade can prevent a leakage flow of the fluid through a clearance between the spiral blade and the casing of the centrifugal compressor. Thus, the pressure of the fluid can be increased by the pre-compression unit without fail, whereby the partial condensation can be more effectively prevented in the radial flow path.

In one embodiment, the centrifugal compressor further includes a sealing member disposed between an outer circumference surface of the shroud and a wall surface of a casing of the centrifugal compressor, the wall surface facing the outer circumference surface.

With the sealing member thus provided, the leakage flow of the fluid through the clearance between the spiral blade and the casing of the centrifugal compressor can be more effectively prevented.

In some embodiments, the impeller is formed separately from the spiral blade and the shroud.

Thus, the impeller and the spiral blade with the shroud can be separately manufactured, and can be processed easily.

Advantageous Effects

In at least one embodiment of the present invention, the pre-compression unit increases the pressure of the fluid before flowing into the radial flow path. Thus, the pressure of the fluid can be easily maintained at or higher than the saturation pressure in the vicinity of the leading edge of the impeller. Thus, the partial condensation can be prevented from occurring. Thus, a high compression efficiency can be maintained while preventing the degradation of the compression performance. All things considered, the centrifugal compressor can operate under an operation condition with a low inlet temperature, and thus the compression efficiency can be further improved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view illustrating a schematic configuration of a centrifugal compressor according to some embodiments.

FIG. 2 is an enlarged view of a main part of a centrifugal compressor according to one embodiment.

FIG. 3 is an enlarged view of a main part of a centrifugal compressor according to another embodiment.

FIG. 4 is a diagram illustrating a configuration of a compression system according to one embodiment.

FIG. 5 is a perspective view illustrating an example of a configuration of an impeller.

FIG. 6 is one example of a T-S diagram.

DETAILED DESCRIPTION

The following describes some embodiments of the present invention with reference to the accompanying drawings. It should be noted that the size, material, shape, relative arrangement, and the like of components described in the embodiments and illustrated in the drawings are given by way of example only and are not intended to limit the scope of the present invention to these.

First of all, a schematic configuration of a centrifugal compressor 1, 1A, 1B according to the present embodiment is described with reference to FIG. 1 to FIG. 3. FIG. 1 is a cross-sectional view illustrating a schematic configuration of a centrifugal compressor according to some embodiments. FIG. 2 is an enlarged view of a main portion of a centrifugal compressor according to one embodiment. FIG. 3 is an enlarged view of a main portion of a centrifugal compressor according to another embodiment.

The centrifugal compressor 1 illustrated in FIG. 1 is a back to back compressor in which an impeller 20, on a third stage (discharge side) in a low-pressure section 4, and an impeller 20, on a third stage (discharge side) in a high-pressure section 5, are arranged with their back sides facing each other. A structure of the centrifugal compressor according to the present embodiment is not limited to this. The low-pressure section 4 and the high-pressure section 5 illustrated in FIG. 1 have approximately the same configuration. Thus, first-stage impellers 20A and 20B in the low-pressure section 4 and peripheral structures thereof are representatively illustrated in FIG. 2 and FIG. 3. In the figures, the same portions are denoted with the same reference numerals.

In the embodiments described below, the multi-stage centrifugal compressors (multi-stage compressors) 1, 1A, and 1B are described as an example.

As illustrated in FIG. 1 to FIG. 3, the centrifugal compressor 1, 1A, 1B according to some embodiments is configured to compress a fluid in a gas phase or a supercritical phase, and mainly includes: a rotational shaft 2; the low-pressure section 4 and the high-pressure section 5 arranged around the rotational shaft 2; and a casing 6 which supports the rotational shaft 2 in such a manner as to be rotatable about an axis.

The rotational shaft 2 is rotatably supported by the casing 6 via a bearing 9. The rotational shaft 2 is configured to be rotated by driving force from an external device such as a motor.

The casing 6 has a column shape and has an outer circumference covered by a cylindrical housing 8. In the casing 6, the rotational shaft 2 is disposed through the center of the casing 6, and a flow path 10 for a fluid as a compression target is formed on an outer circumference side of the rotational shaft 2.

The low-pressure section 4 and the high-pressure section 5 each include the flow path 10 and the impellers 20, 20A, and 20B. The low-pressure section 4 and the high-pressure section 5 may each include a diffuser 29 disposed on a downstream side of the impeller 20, 20A, 20B. The diffuser 29 is configured to convert kinetic energy, provided to the fluid by the impeller 20, 20A, 20B, into pressure energy.

The flow path 10 includes: an inlet port 11 and a discharge port 17 formed in the casing 6 and the housing 8; an axial flow path 13 formed in the casing 6; and a radial flow path 14 communicating with the axial flow path 13.

For example, the flow path 10 has the following specific configuration. Specifically, the inlet port 11, a straight flow path 12, the axial flow path 13, the radial flow path 14, a return flow path 15, a straight flow path 16, and a discharge port 17, communicating with each other, are arranged in this order from an upstream side to the downstream side.

The straight flow path 12, communicating with the inlet port 11, linearly extends in the radial direction of the centrifugal compressor I, 1A, 1B. A fluid sucked in through the inlet port 11 flows in the radial direction from a radially outer side toward a radially inner side of the centrifugal compressor 1, 1A, 1B, while passing through the straight flow path 12.

The axial flow path 13 extends along an axial direction of the centrifugal compressor 1, 1A, 1B. The axial flow path 13 may extend along the axial direction of the rotational shaft 2. The axial flow path 13 has an upstream end side communicating with the straight flow path 12 via a corner area, and has a downstream side communicating with the radial flow path 14. The axial flow path 13 is configured in such a manner that a fluid flowing in, with a flow direction converted from that in the straight flow path 12, toward the radially inner side, into a direction along the axial direction, flows along the axial direction for a predetermined distance.

The radial flow path 14, communicating with the axial flow path 13, extends along the radial direction of the centrifugal compressor 1, 1A, 1B, on the downstream side of the axial flow path 13. The radial flow path 14 has the impeller 20, 20A, 20B disposed on the upstream side (inner circumference side) and the diffuser 29 disposed on the downstream side (outer circumference side). The radial flow path 14 is configured in such a manner that in an upstream side compression area including the impeller 20, 20A, 20B, the fluid flow that has flowed along the axial direction in the axial flow path 13 has the flow direction converted into a direction toward the radially outer side and is compressed by the impellers 20, 20A, and 20B.

The return flow path 15 has an approximately U-shaped cross section, and has an upstream end side communicating with the radial flow path 14 and a downstream end side communicating with the straight flow path 16. The return flow path 15 is configured in such a manner that the fluid, which has passed through the impeller 20, 20A, 20B and flowed toward the radially outer side in the radial flow path 14, has the flow direction reversed to flow toward the radially inner side, and thus is sent to the straight flow path 16.

The straight flow path 16 has an upstream end side communicating with the return flow path 15 and a downstream end side communicating with the axial flow path 13 in the subsequent stage.

The fluid that has passed through the impellers 20, 20A, and 20B on all the stages passes through the straight flow path 16 in the final stage and then is discharged through the discharge port 17.

The impeller 20, 20A, 20B is arranged on at least one stage along the axial direction of the rotational shaft 2. The configuration exemplarily illustrated in FIG. 1 includes the impellers 20, 20A, and 20B on three stages, including a first-stage impeller. In this configuration, a plurality of (three stages in this example) the impellers 20, 20A, and 20B are arranged at an interval along the axial direction of the rotational shaft 2.

The impeller 20, 20A, 20B on each stage is at least partially disposed in the radial flow path 14, and is configured to rotate together with the rotational shaft 2 to increase the pressure of the fluid flowing in the radial flow path 14. The impeller 20, 20A, 20B on each stage includes: a hub 21 which has a disk shape and is fixed to an outer circumference of the rotational shaft 2; and a plurality of blades (vanes) 22 fixed and radially arranged on the hub 21. The compression area of the radial flow path 14 is formed of a space defined by the hub 21 and the adjacent blades 22.

The centrifugal compressor 1A according to the embodiment illustrated in FIG. 2 has no shroud covering the impeller 20A.

The centrifugal compressor 1B according to the embodiment illustrated in FIG. 3 further includes a shroud 27 covering the impeller 20B mainly for improving sealability of the flow path 10. The shroud 27 is attached to a distal end of each of the blades 22 of the impeller 20B, and is concentrically arranged with the rotational shaft 2. In this embodiment, the compression area of the radial flow path 14 is formed of a space defined by the hub 21, the adjacent blades 22, and the shroud 27. A gap between the shroud 27 and the casing 6 may be provided with a sealing member 28 which prevents the fluid from leaking.

FIG. 4 is a diagram illustrating a configuration of a compression system 100 according to one embodiment.

The compression system 100 according to one embodiment includes: a low-pressure compressor 101A; a mid-pressure compressor 101B; a high-pressure compressor 101C; and a cooler group 40 including coolers 41 to 44. At least one of the low-pressure compressor 101A, the mid-pressure compressor 101B, and the high-pressure compressor 101C has the same configuration as the centrifugal compressor 1, 1A, 1B described above.

The low-pressure compressor 101A includes a first section 4A on a low-pressure side and a second section 5A on a high-pressure side. The mid-pressure compressor 101B includes a third section 4B on the low-pressure side and a fourth section 5B on the high-pressure side. The high-pressure compressor 101C includes a fifth section 4C on the low-pressure side and a sixth section 5C on the high-pressure side. Based on the example of the configuration illustrated in FIG. 1, the first section 4A, the third section 4B, or the fifth section 4C corresponds to the low-pressure section 4, and the second section 5A, the fourth section 5B, or the sixth section 5C corresponds to the high-pressure section 5.

In the compression system 100, the fluid compressed by the first section 4A in the low-pressure compressor 101A is cooled by the cooler 41, further compressed in the second section SA, and then is sent to the cooler 42. Next, in the mid-pressure compressor 101B, the fluid cooled by the cooler 42 is introduced to the third section 4B, and the fluid compressed in the third section 4B is cooled in the cooler 43. The fluid after the cooling is further compressed in the fourth section 5B, and then is sent to the cooler 44. Next, in the high-pressure compressor 101C, the fluid cooled by the cooler 44 is introduced to the fifth section 4C, compressed in the fifth section 4C, further compressed in the sixth section 5C, and then is discharged.

It is a common practice to cool the fluid with the cooler group 40 to improve the compression efficiency to reduce the driving force required in the compression system 100 as described above. However, when the compressor inlet temperature of the fluid is too low, portions at or below saturation pressure might be locally generated in the compressor and thus partial condensation might occur.

As a result of vigorous studies, the present inventors have found out that the partial condensation, as a factor of degrading the performance of the centrifugal compressor 1, 1A, 1B, is likely to occur in an area 50 on an inlet side of the impeller 20 as illustrate in FIG. 5. FIG. 5 is a perspective view illustrating an example of a configuration of the impeller 20. Specifically, the blades 22 of the impeller 20 each include a leading edge 23, a trailing edge 24, a pressure surface 25, and a suction surface 26. As a result of CFD (computational fluid dynamics), the present inventors have found out that a flow rate tends to increase in the vicinity of the leading edge 23 of the impeller 20 (the blade 22 herein), that is, in the area 50 of the leading edge 23 in the vicinity of the suction surface 26 in particular. Based on this tendency, it has been found that static pressure, around the impeller 20, is the lowest in the vicinity of the leading edge 23 of the impeller 20 (more specifically, in the vicinity of the leading edge 23 on the suction surface 26 of the impeller 20), while the centrifugal compressor 1, 1A, 1B is operating. Thus, a risk of partial condensation is high in this portion. All things considered, it is important to prevent the partial condensation from occurring in a leading edge portion of the impeller 20 to improve the compression efficiency of the centrifugal compressor 1, 1A, 1B with the compressor inlet temperature set to be low.

In view of the above, the centrifugal compressor 1, 1A, 1B according to the present embodiment further has the following configuration to improve the compression efficiency while preventing the partial condensation in the compressor.

As illustrated in FIG. 1 to FIG. 3, in some embodiments, the centrifugal compressor 1, 1A, 1B further includes a pre-compression unit 30, 30A, 30B.

The pre-compression unit 30, 30A, 30B is disposed in the axial flow path 13 at a position distant from the leading edge 23 of the impeller 20 on an upstream side of the leading edge 23. The pre-compression unit 30 is configured to rotate together with the rotational shaft 2 about the axis to increase the pressure of the fluid flowing in the axial flow path 13. The pre-compression unit 30 is formed separately from the impeller 20.

In the embodiment described above, the fluid is pre-compressed by the pre-compression unit 30, 30A, 30B disposed in the axial flow path 13 on an upstream side of the leading edge 23 of the impeller 20. Thus, the pressure of the fluid can be maintained at or above the saturation pressure in the vicinity of the leading edge 23 of the impeller 20, whereby the partial condensation can be prevented from occurring. Thus, a high compression efficiency can be maintained while preventing the compression performance from degrading.

The partial condensation might occur in the pre-compression unit 30, 30A, 30B disposed in the axial flow path 13. Still, the centrifugal compressor 1, 1A, 1B is less affected by such partial condensation compared with the partial condensation occurring in the radial flow path 14 for the following reason. Specifically, the flow direction of the fluid and the direction of the centrifugal force are different from each other in the axial flow path 13. Thus, droplets as a result of the partial condensation in the axial flow path 13 would not spread over the entire axial flow path 13 due to the centrifugal force. On the other hand, droplets as a result of the partial condensation on the inlet side of the radial flow path 14 would spread over the entire radial flow path 14 due to the centrifugal force, and might block the flow path 10.

Conventionally, it has been a common practice to set the compressor inlet temperature to be much higher than a theoretical condensation temperature in the compression system 100 as illustrated in FIG. 4, due to the risk of partial condensation. As illustrated in a T-s diagram (temperature-entropy diagram) in FIG. 6, in theory, the condensation should not occur with a temperature in a region higher than a saturation liquid line 52 including a critical point. Still, due to the risk of the partial condensation, an operation line 53 of the inlet temperature has been set to be much higher (on a high temperature side) than the saturation liquid line 52.

In the centrifugal compressor 1, 1A, 1B according to the embodiment described above, the partial condensation can be prevented from occurring. Thus, for example, in the fourth section 5B (see FIG. 4), the inlet temperature to be set can be lowered down to an operation line 54. Thus, the power required for driving the centrifugal compressor 1, 1A, 1B can be largely reduced, whereby the compression efficiency of the centrifugal compressor can be improved.

As described above, the pre-compression unit 30, 30A, 30B disposed in the axial flow path 13 increases the pressure of (pre-compresses) the fluid. Thus, even when the inlet temperature of the centrifugal compressor 1, 1A, 1B is low, the pressure in the vicinity of the leading edge 23 of the impeller 20 can be maintained at or higher than the saturation pressure, whereby the partial condensation can be prevented from occurring. Thus, the centrifugal compressor 1, 1A, 1B can be operated under an operation condition with a lower inlet temperature, that is, under an operation condition which has had a high risk of partial condensation in conventional configurations. Thus, the compression efficiency can be improved.

In some embodiments, the centrifugal compressor 1, 1A, 1B is a multi-stage compressor (see FIG. 1) including at least one section 4, 5 including the plurality of impellers 20, 20A and 20B arranged on multiple stages along the flow direction of the fluid. The pre-compression unit 30, 30A, 30B is disposed in the axial flow path 13, on the upstream side of the first-stage impeller 20, 20A, 20B in each section 4, 5, at a position distant from the leading edge 23 of the first-stage impeller 20, 20A, 20B on an upstream side of the leading edge 23.

In the multi-stage compressor, the flow path in the vicinity of the first-stage impeller 20, 20A, 20B has a lower pressure than flow paths in the vicinity of other impellers, and thus is regarded as having a highest risk of the partial condensation. Thus, as in the embodiment described above, the pre-compression unit 30, 30A, 30B is disposed in the axial flow path on the upstream side of the first-stage impeller 20, 20A, 20B, and pre-compresses the fluid before flowing into the radial flow path 14 in the vicinity of the leading edge 23 of the first-stage impeller 20, 20A, 20B. Thus, the partial condensation can be effectively prevented in the area (in the vicinity of the leading edge of the first-stage impeller) where the condensation is most likely to occur.

In one embodiment, the axial flow path 13 is configured to linearly extend along the axial direction of the centrifugal compressor 1, 1A, 1B, and has a predetermined distance. For example, the distance of the axial flow path 13 in the axial direction is not shorter than a blade height at the leading edge 23 of the impeller 20, 20A, 20B.

As illustrated in FIG. 2 and FIG. 3, in one embodiment, the pre-compression unit 30A, 30B includes a spiral blade 31A, 31B which is disposed on an outer circumference side of the rotational shaft 2 and extends along the axial direction spirally around the rotational shaft 2.

In the embodiment described above, when the rotational shaft 2 rotates, the spiral blade 31A, 31B rotates so that a fluid G₁ flows into the spiral blade 31A, 31B in the axial flow path 13. With the spiral blade 31A, 31B, the fluid G₁ is guided toward the radial flow path 14 while having the pressure increased. A fluid G₂ that has passed through the spiral blade 31A, 31B has a higher pressure than the fluid G₁ before passing through the spiral blade 31A, 31B. With the spiral blade 31A, 31B thus used for the pre-compression unit 30A, 30B, the pressure of the fluid can be increased by the driving force of the rotational shaft 2, whereby a simple device configuration can be achieved.

The pre-compression unit 30A, 30B may include a cylindrical portion (not illustrated) disposed to surround the outer circumference surface of the rotational shaft 2, and the spiral blade 31A, 31B may be provided on the cylindrical outer circumference surface. Thus, the pre-compression unit 30A, 30B can be more effectively fit to the rotational shaft 2.

In one configuration example, at least a part of the axial flow path 13 is defined by the spiral blade 31A, 31B and the casing 6. More specifically, in an area of the axial flow path 13 where the pre-compression unit 30A, 30B is positioned, the casing 6 is not provided on the outer circumference of the rotational shaft 2. Thus, in this area, the outer circumference surface of the rotational shaft 2 is exposed to the axial flow path 13. The pre-compression unit 30A, 30B (spiral blade 31A, 31B) is attached to the outer circumference surface of the rotational shaft 2 exposed to the axial flow path 13. In this configuration, the pre-compression unit 30A, 30B, rotating together with the rotational shaft 2, can be easily attached.

As illustrated in FIG. 3, in the centrifugal compressor 1B according to the other embodiment, the pre-compression unit 30B further includes a shroud 32 disposed on an outer circumference side of the spiral blade 31B to cover the spiral blade 31B. For example, the shroud 32 is formed to have an annular shape around an axis O of the rotational shaft 2. The shroud 32 and the spiral blade 31B may be integrally formed. For example, the shroud 32 is attached to the outer circumference surface of the spiral blade 31B, and is configured to rotate together with the spiral blade 31B fixed to the rotational shaft 2. In this case, the shroud 32 and the spiral blade 31B may be formed as different members, and the members may be integrated by welding or the like.

In the embodiment described above, the shroud 32 disposed on the outer circumference side of the spiral blade 31B can prevent a leakage flow of the fluid through a clearance between the spiral blade 31B and the casing 6 of the centrifugal compressor 1B. Thus, the pressure of the fluid can be increased by the pre-compression unit 30B without fail, whereby the partial condensation can be more effectively prevented in the radial flow path 14.

A sealing member 33 disposed between an outer circumference surface of the shroud 32 and a wall surface of the casing 6 of the centrifugal compressor 1B, facing the outer circumference surface, may be further provided. More specifically, the sealing member 33 is formed to have an annular shape, and is disposed between an inner wall of the casing 6 facing the shroud 32 and the outer circumference surface (back surface) of the shroud 32. The sealing member 33 may be provided in an upstream side area of the axial flow path 13. For example, the sealing member 33 having an annular shape may be accommodated in a groove portion (not illustrated) that has an annular shape around the axis O of the rotational shaft 2 and is formed on at least one of the outer circumference surface of the shroud 32 and the wall surface of the casing 6.

With the sealing member 33 thus provided, the leakage flow of the fluid through the clearance between the spiral blade 31B and the casing 6 of the centrifugal compressor 1B can be more effectively prevented.

The impeller 20B may be formed separately from the spiral blade 31B and the shroud 32. Thus, the impeller 20B and the spiral blade 31B with a shroud can be separately manufactured, and can be processed easily.

In the above described embodiment of the present invention, the pre-compression unit 30, 30A, 30B pre-compresses the fluid before flowing into the radial flow path 14. Thus, the pressure of the fluid can be easily maintained at or higher than the saturation pressure in the vicinity of the leading edge 23 of the impeller 20, 20A, 20B. Thus, the partial condensation can be prevented from occurring. Thus, a high compression efficiency can be maintained while preventing the degradation of the compression performance. All things considered, the centrifugal compressor 1, 1A, 1B can operate under an operation condition with a low inlet temperature, and thus the compression efficiency can be further improved.

The present invention is not limited to the embodiment described above, and includes a mode obtained by modifying the embodiment described above, and a mode obtained by appropriately combining the modes.

In the embodiment described above, the multi-stage centrifugal compressor (multi-stage compressor) 1, 1A, 1B is described as an example. A part of the configuration according to the present embodiment can be applied to a compressor with a single stage (single-stage compressor).

In the configuration of the embodiment described above, the pre-compression unit 30, 30A, 30B includes the spiral blade 31A, 31B. However, the pre-compression unit 30, 30A, 30B is not limited to this configuration. The configuration of the pre-compression unit 30, 30A, 30B is not particularly limited as long as the pre-compression unit 30, 30A, 30B is disposed in the axial flow path 13 and can pre-compress the fluid.

For example, the expressions used herein that mean relative or absolute arrangement, such as “in a direction”, “along a direction”, “in parallel with”, “orthogonal with”, “center”, and “concentrically”, mean not only exactly what they refer to but also such states that are relatively displaced with a tolerance or by an angle that is small enough to achieve the same level of functionality.

The expressions used herein that mean things are equivalent to each other, such as “the same”, “equivalent”, and “uniform”, mean not only exactly equivalent states but also such states that have a tolerance or a difference that is small enough to achieve the same level of functionality.

For example, expressions that represent shapes, such as quadrangles and cylinders, mean not only what they refer to in a geometrically strict sense but also shapes having some irregularities, chamfered portions, or the like that can provide the same level of functionality.

The expressions “including”, “comprising”, and “provided with” one component are not exclusive expressions that exclude other components.

REFERENCE SIGNS LIST

1, 1A, 1B Centrifugal compressor 2 Rotational shaft 4, 4A to 4C Low-pressure section 5, 5A to 5C High-pressure section

6 Casing

10 Flow path 11 Inlet port 13 Axial flow path 14 Radial flow path 17 Discharge port

20, 20A, 20B Impeller 21 Hub 22 Blade 27 Shroud

28 Sealing member

29 Diffuser

30, 30A, 30B Pre-compression unit 31, 31A, 31B Spiral blade

32 Shroud

33 Sealing member 52 Saturation liquid line 53 Operation line 100 Compression system

O Axis 

1. A centrifugal compressor for compressing a fluid in a gas phase or a supercritical phase, the centrifugal compressor comprising: a rotational shaft; an axial flow path extending along an axial direction of the centrifugal compressor; a radial flow path communicating with the axial flow path and extending along a radial direction of the centrifugal compressor on a downstream side of the axial flow path; an impeller at least partially disposed in the radial flow path and configured to rotate together with the rotational shaft to increase pressure of the fluid flowing in the radial flow path; and a pre-compression unit disposed in the axial flow path at a position distant from a leading edge of the impeller on an upstream side of the leading edge and configured to increase the pressure of the fluid in advance before the fluid is introduced to the leading edge, the centrifugal compressor being a multi-stage compressor including at least one section including a plurality of the impellers arranged on multiple stages along a flow direction of the fluid, and the pre-compression unit being disposed in the axial flow path on an upstream side of a first-stage impeller in each of the at least one section, at a position distant from a leading edge of the first-stage impeller on an upstream side of the first-stage impeller.
 2. (canceled)
 3. The centrifugal compressor according to claim 1, wherein the pre-compression unit is configured to rotate together with the rotational shaft to increase the pressure of the fluid.
 4. The centrifugal compressor according to claim 1, wherein the pre-compression unit includes a spiral blade disposed on an outer circumference side of the rotational shaft and extending along the axial direction spirally around the rotational shaft.
 5. The centrifugal compressor according to claim 4, wherein the pre-compression unit further includes a shroud disposed on an outer circumference side of the spiral blade so as to cover the spiral blade.
 6. The centrifugal compressor according to claim 5, further comprising a sealing member disposed between an outer circumference surface of the shroud and a wall surface of a casing of the centrifugal compressor, the wall surface facing the outer circumference surface.
 7. The centrifugal compressor according to claim 5, wherein the impeller is formed separately from the spiral blade and the shroud. 